1. Field of the Invention
This invention relates to optical switches and more specifically to a monolithic 2D optical switch using MEMS technology.
2. Description of the Related Art
The network of copper wires that has been the backbone of the telecommunications network is rapidly being replaced with a fiber optic network to increase the bandwidth available to support the Internet and other networking applications. To date, the majority of the original long haul telephone copper network has been replaced with an optical fiber network and network links within metropolitan areas are rapidly being replaced. While this copper to fiber replacement is proceeding at a breakneck pace, the demand for high bandwidth communication is so great that the rate of replacement will accelerate.
All communication networks, either copper or optical fiber, require switches that can route signals from source to destination as well as re-route signals in case of a fault or an excessive demand for a specific link. Presently the switching in xe2x80x9clong haulxe2x80x9d and xe2x80x9cmetroxe2x80x9d segments of fiber optic networks is done electronically. The optical signals are converted into electronic signals and then electronic switching matrices, similar to the ones used in the original copper network, are used to accomplish the routing. After routing, the electronic signals are converted back to an optical signal and sent out through the designated fiber. This type of xe2x80x9copticalxe2x80x9d routing switch is large, expensive and inefficient. The electronic components of this type of switch are the major bottleneck in overall network throughput capacity.
The rapid growth in the number of fiber optic lines has created an urgent need for an all optical router; one that does not need to transform the signal into an electronic signal. An optical crossbar switch, routes N incoming fiber optic channels to N outgoing fiber optic channels by selective actuation of a micro-mirror array to alter the desired light paths. These MEMS based optical crossbar switches should be capable of routing more channels on a single device, and be far cheaper and more compact than opto-electronic switches.
To date MEMS crossbar switches have not fulfilled their promise. The current switch designs and the limited manufacturing yields have constrained the size of useful devices typically to 2xc3x972. Although these small devices can be cascaded together to form a larger device, such a cascaded configuration is complicated, lossy and very expensive. Some of the key problems have, and continue to be, the inability to precisely control the deflection angles of the micro-mirrors, to reduce the footprint of the actuation mechanism, and to monolithically fabricate the MEMS structures on an integrated circuit (IC).
Two main categories of MEMS optical crossbar switches exist. The first is based on sliding a vertical mirror in and out of a light path to perform a switching function. Lucent Technologies, Inc., U.S. Pat. No. 5,923,798 proposed an xe2x80x9cin-planexe2x80x9d optical switch that includes an actuator comprising two vertically-oriented electrodes, and a linkage from the actuator to an optical device. As a voltage is applied across the electrodes, the movable electrode swings toward the fixed electrode. The horizontal displacement of the electrode is transferred to the optical devices which moves in-plane in or out of the optical path.
Lucent Technologies, Inc., U.S. Pat. No. 5,995,688, also proposed a micro-opto-electromechanical devices performing xe2x80x9con-offxe2x80x9d switching function for only one optical channel. The MEMS device comprises an actuator that is mechanically linked to an optical interrupt (e.g., micro-mirror). The first end of the linkage underlies and abuts a portion of movable plate electrode, and a second end of linkage supports optical interrupter. The interrupter is a vertically assembled mirror that is attached to the linkage. When a voltage is applied across plate actuator, an electrostatic attraction causes a vertical or out-of-plane motion to linkage such that optical interrupter moves xe2x80x9cup-and-downxe2x80x9d. In an actuated state, the device causes the optical interrupt of an optical signal. This device can be practically used only as one channel chopper.
The second category of switches, which is also the subject of this patent, is based on hinged mirrors that can be rotated out of the plane of the substrate to a vertical position to perform the switching function by selectively blocking the light path. Various mechanisms exist to provide the actuation force necessary to rotate the hinged mirrors including magnetic, thermal and electrostatic. Electrostatic actuation includes both lateral comb drive (in-plane) actuation and parallel-plate (out-of-plane) actuation. Lateral comb drives are used in combination with scratch drives, stepper motors, linear micro-vibrometers and micro-engines.
H. Toshiyoshi et al. xe2x80x9cElectrostatic micro torsional mirrors for an optical switch matrix,xe2x80x9d IEEE J. Microelectromechanical System, Vol. 5, no. 4, pp. 231-237, 1996 describes a free-space optical switch based on parallel-plate actuation. The device is composed of two parts: torsion mirror substrate (a) and counter electrode substrate (b). As shown in FIG. 5, a bulk micromachining process is used to fabricate the mirror substrate in which a matrix of micro mirrors are supported by torsion beams across respective through-holes etched into the backside of the substrate. Bulk micromachining is relatively slow, expensive, provides only nominal control of mirror thickness and flatness, and is not compatible with IC fabrication processes yet. The mirror and counter electrode substrates are manually aligned by microscope observation and fixed by putting epoxy glue on the edge.
Application of a bias voltage to the mirror and counter electrodes attracts the mirror inward by 90xc2x0 to reflect the incident light. The incident and redirected lights can propagate through the deep grooves formed on the backside of the substrate; i.e., the mirrors are located at the crossings by 45xc2x0 inclination to the grooves. The angle of the mirror in the ON-state (90xc2x0) is controlled because it touches a mechanical stopper on the counter substrate.
The stiction force between the mirror and stopper creates a hysteresis whereby the applied voltage can be reduced and yet be able to hold the mirror in the ON-state. The spring force of the hinged mirror must be sufficient to overcome the stiction force when the holding voltage is completely removed in order to return the mirror to the OFF-state. Consequently, the applied voltage must be sufficient to overcome the mirror""s spring force to drive the mirror to the ON-state, approximately 100-150V, which is not compatible with either standard IC processing or off-the-shelf driver chips.
Although the switch configuration may, in theory, be extended to arbitrary sizes it will in practice be limited to small devices on the order of 2xc3x972. The combination of a mechanical stop, bulk processing and manual assembly of the mirror and counter electrode substrates limits the precision of the mirror deflection angle in the ON-state. The small (input) acceptance angle of the output fiber forces a high degree of precision on the deflection angle. This in turn determines the maximum path length between an input fiber and an output fiber, hence the array size. In addition, array size is limited by space considerations owing to the fact that a lead must be provided for each mirror in each row or column and the actuator footprint.
In view of the above problems, the present invention provides a free-space micromachined 2D optical switch with improved precision, hence larger array sizes at lower cost, whose fabrication is compatible with standard IC fabrication. The 2D optical switch will find particular use in an all-optical fiber network and, even more specifically, in the last mile of the network.
This is accomplished using a sequence of MEMS processing steps to construct on a single substrate an array of hinged micromirrors that are supported by an array of posts to allow deflection of the mirrors through approximately a 90xc2x0 rotation under the influence of an electrostatic field between the mirrors and their underlying control electrodes.
In a typical optical switch configuration the mirrors are oriented at a 45xc2x0 angle to the input and output optical paths and positioned parallel to the substrate either above, below or, perhaps, in the optical paths. The application of a voltage between the mirror and its control electrodes switches the mirror to a vertical position where it intercepts and deflects light travelling down the optical paths. Reproducibly accurate positioning of the mirrors requires either the use of active positioning control or of mechanical stops. The posts are suitably oriented at a 90xc2x0 angle with respect to the mirror hinges so that they do not interfere with the optical paths and, may be configured to function as baffles to reduce crosstalk between adjacent optical paths.